Are you talking about complete synchronization of the OS filesystems? I don't know any way to do that on-line. You can back up each system to available storage on the other. You can also synchronize user data files by various methods. Perhaps you are interested in a [url=http://www.google.com/search?q=High+Availability+%28HA%29+cluster&ie=utf-8&oe=utf-8&aq=t&rls=org.mozilla:en-US:official&client=firefox-a&safe=active]High Availability (HA) cluster[/url]. Please clarify your requirements.